[Week 4–7] GraphNet EC solver: benchmark against Ridge on real HCP connectomes

[TheRoy666]

GraphNet FISTA solver is implemented and unit-tested. This issue tracks
the scientific validation of the solver on real structural connectomes.

Tasks:

• Compare GraphNet EC vs Ridge EC on HCP S1200 subject (N=400, T=1200)
• Sweep lambda_graph (0.1, 1.0, 5.0, 10.0) and measure SC-EC coupling r
• Benchmark FISTA convergence speed at N=400 vs N=20 (scaling test)
• Verify Teleportation Error ratio holds on real HCP data (expect >> 1.6x)
• Document recommended lambda_ridge / lambda_graph for HCP Schaefer-400

Tracks GSoC Weeks 4–7 deliverable.